A variety of polymeric materials have been utilized as electrical insulating materials for power cables and other electrical devices. Typical insulation compounds include elastomers such as ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM), collectively referred to herein as EP(D)M. These insulation compounds are applied as an insulation member over either a metallic conductor or a semi-conductive substrate in a multi-step extrusion process.
EP(D)M polymers used in electrical applications generally contain fillers within the range of from 40 to 100 parts per hundred parts by weight of rubber (phr) to achieve acceptable mechanical properties and extrusion processability. The addition of filler, however, increases power loss through the cable. In the power transmission and distribution industry, power loss is associated with cost debits. The cost associated with power loss is proportional to the voltage, and becomes significant in medium voltage applications (5 to 69 kV) and even more significant in high voltage applications (>69 kV).
U.S. Pat. No. 5,674,613 discloses filled electrical insulation compounds comprising an ethylene, propylene, 5-vinyl-2-norbornene elastomeric polymer having a branching index up to 0.4 and a Mw/Mn greater than 10 and an elastomeric polymer of an ethylene-α-olefin copolymer having a Mw/Mn less than 3, a CDBI greater than 50% and a density in the range of from 0.86 to 0.92 g/cm3.
U.S. Pat. No. 6,270,856 discloses an electrical insulating layer comprising an ethylene-α-olefin copolymer, optionally including a diene, and having a density of from about 0.86 to about 0.96 g/cm3, a melt index of from about 0.2 to about 100 dg/min, a molecular weight distribution of from about 1.5 to about 30, and a composition distribution breadth index greater than about 45%. The insulating layer may contain neat (i.e., unfilled) or filled polymer.
Semicrystalline polypropylenes having at least 75 wt % propylene-derived units and a triad tacticity of greater than 75% are known. See, for example, U.S. Pat. No. 6,525,157. U.S. Patent Application Publication No. 2004-0059061 describes semicrystalline polypropylenes blended with EP(D)M to form one component of a multiple component structure useful in, for example, automobile glass run channels and door seals.